Some vehicle engine systems utilizing direct in-cylinder injection of fuel include a fuel delivery system that has multiple fuel pumps for providing suitable fuel pressure to fuel injectors. This type of fuel system, Gasoline Direct Injection (GDI), is used to increase the power efficiency and range over which the fuel can be delivered to the cylinder. GDI fuel injectors may require high pressure fuel for injection to create enhanced atomization for more efficient combustion. As one example, a GDI system can utilize an electrically driven lower pressure pump (i.e., a fuel lift pump) and a mechanically driven higher pressure pump (i.e., a direct injection pump) arranged respectively in series between the fuel tank and the fuel injectors along a fuel passage. In many GDI applications the high-pressure or direct injection fuel pump may be used to increase the pressure of fuel delivered to the fuel injectors. The high-pressure fuel pump may include a solenoid actuated “spill valve” (SV) or fuel volume regulator (FVR) that may be actuated to control flow of fuel into the high-pressure fuel pump. Various control strategies exist for operating the higher and lower pressure pumps to ensure efficient fuel system and engine operation.
In one approach to control the direct injection fuel pump, shown by Cinpinski and Lee in U.S. Pat. No. 7,950,371, a diagnostic module controls a fuel pump module to operate a fuel pump that provides fuel to a fuel rail. The diagnostic module determines a predetermined amount of fuel to send to the fuel rail, determines an estimated pressure increase within the fuel rail based on the predetermined amount of fuel, and compares an actual pressure increase to an estimated pressure increase. Based on the comparison, the fuel pump control module selectively controls the fuel pump. In an example control scheme for operating the high pressure (direct injection) fuel pump, several steps are performed to compensate the fuel rail pressure in order to bring an actual rail pressure increase closer to an estimated rail pressure increase. Several steps involve measuring rail pressure and comparing that value to a threshold, upon which a commanded increase in pressure via operation of the fuel pump is monitored.
However, the inventors herein have identified potential issues with the approach of U.S. Pat. No. 7,950,371. First, while the control method of Cinpinski and Lee may provide control of the direct injection fuel pump to maintain operation near a desired threshold pressure, the method does not address several issues that may arise with lower pump displacement volumes. Lower pump displacement volumes may range from about 0% to 40% depending on the particular fuel system, wherein the percentage refers to the percentage of total pump displacement compressed and sent to the attached fuel rail. With lower displacement volumes, control of the direct injection pump (via the spill valve) may be inaccurate and variable. Therefore, the quantity of fuel pumped into the fuel rail may be unknown while commanding lower displacement volumes with low accuracy. As such, diagnostic and control functions may not be executed properly due to the variability in pump control.
Thus in one example, the above issues may be at least partially addressed by a method, comprising: when a calculated pump command of a direct injection fuel pump is between 0 and a zero flow lubrication command, issuing the zero flow lubrication command to a solenoid spill valve of the fuel pump; when the calculated pump command is between the zero flow lubrication command and a threshold command, issuing the threshold command; and when the calculated pump command is greater than the threshold command, issuing the calculated pump command. In this way, the direct injection pump is operated outside the regions where low accuracy and variable pump commands occur. Due to this, the pump may be only operated in regions and at commands where accurate and repeatable control is more likely to occur. Since fuel and engine systems vary between vehicles, the control method can be adjusted to learn what the zero flow lubrication and threshold commands are for a specific configuration. Issuing the zero flow lubrication command may accomplish the desired result of transferring no fuel into the fuel rail while creating a pressure difference across the pump piston which forces liquid into the piston-bore interface, thereby lubricating the piston-bore interface.
In another example, the issued direct injection pump commands depend on whether or not a measured fuel rail pressure is less than or greater than a desired fuel rail pressure. If the measured fuel rail pressure is less than the desired fuel rail pressure, then the issued pump commands are determined as described above. Alternatively, if the measured fuel rail pressure is greater than the desired fuel rail pressure, then the direct injection fuel pump is operated at the zero flow lubrication command. As explained in further detail later, the zero flow lubrication command may correspond to an energized time period of the solenoid spill valve that defines the boundary between 0 fuel volume pumped and a greater-than-0 fuel volume pumped. The pump commands cause specific pump trapping volumes to occur. Pump trapping volume, or displacement or pumped volume, is a measure of how much fuel is compressed and ejected to a fuel rail by the direct injection fuel pump.
In one example control strategy, the threshold command is chosen such that if the preliminary DI pump command is between the ZFL command and threshold command, the threshold command is issued. While this control strategy adds more fuel to the fuel rail than otherwise desired, the fuel pumped amount is increased to a less-variable level. As such, the control strategy effectively forms a minimum volume pumped into the fuel rail. Having a predictable fuel amount pumped may be beneficial for fuel rail pressure control and aid in vapor detection at the DI fuel pump inlet. Aiding in fuel vapor detection may result from the fuel pressure increase becoming measurable when it is sufficiently large, that is, by clipping the pump commands to the threshold command. As a percent-of-value, small pump volumes may be highly-variable, and therefore small pump volumes (i.e., pump stokes) may be undesirable.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.